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Threatened species total 'hugely underestimated'

The true number of species at risk of extinction could be 50% higher than the total shown on the International Conservation Union’s Red List, according to a bleak new assessment.

That is because existing estimates fail to take into account many species, such as parasites and beetles, that depend for their survival on animals and plants that are recognised as threatened, says the team, led by Lian Pin Koh at the National University of Singapore and Robert Dunn at Curtin University of Technology in Perth, Western Australia.

They estimate that in addition to the 12,200 species on the Red List, at least another 6300 are “co-endangered”. For instance, at least three species of lice depend on the threatened red colobus monkey. Many of these other dependent species might have been overlooked in the past because they are often less charismatic than their hosts, the team says.

“Now, we would like to see changes to the Red List, so that when, for example, a mammal species is listed as endangered, in another column all the species affiliated to that animal are also given, and listed as being possibly at risk,” says Navjot Sodhi, of the National University of Singapore, who was part of the research team.

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Wasps and mites

Craig Hilton-Taylor, an IUCN Red List Programme Officer in Cambridge, UK, welcomes the research, and agrees that it “certainly does elevate the absolute magnitude of recent extinctions and potentially imminent ones”.

But it will not be easy to add details about dependent species to the Red List, because “in the vast majority of cases we just don’t know about a species’ affiliates”, he adds.

The team started out by looking at eight well-studied groups of host animals and their affiliated species, such as the Ficus plant and the Ficus wasp, primate parasites and their hosts, and parasitic mites and lice that live on seabirds.

For each group, they estimated the number of affiliate species that would be expected to survive, given a decreasing number of available host species. Then they estimated the number of affiliate species that would be likely to go extinct with any given number of host extinctions.

Historical extinctions

Based on the results from this wide range of species, they developed a model to estimate levels of co-extinctions for other animals and plants, for which only the average number of hosts for each affiliated species is known. Then they turned to the Red List.

In addition to 399 now-extinct host species of plants, fish, birds and mammals on the Red List, another 200 affiliated species have probably also been lost historically, they estimate.

And of the 9536 currently endangered host species on the Red List for which there is sufficient data to run their model, another 6300 affiliate species are likely to become extinct if their hosts become extinct. This co-endangered list includes more than 4000 beetles, as well as butterflies, lice and other parasites.

Thomas Brooks of the Center for Applied Biodiversity Science of Conservation International in Washington thinks the new research is a significant step forward. “The team has done the conservation community a great favour by synthesising the theory and data on the co-extinction crisis.”